Hydromorphological Adjustment in Meandering River Systems and the Role of Flood Events
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Sediment Dynamics and the Hydromorphology of Fluvial Systems (Proceedings of a symposium held in Dundee, UK, July 2006). IAHS Publ. 306, 2006. 127 Hydromorphological adjustment in meandering river systems and the role of flood events JANET HOOKE Department of Geography, University of Portsmouth, Buckingham Building, Lion Terrace, Portsmouth PO1 3HE, UK [email protected] Abstract This paper takes a selection of meandering rivers in the UK to address issues of morphological adjustment and change. The occurrence of peak flow events over recent decades is analysed for patterns and trends. Certain hydrological phases are identifiable. Changes in morphology and activity of the channels are examined through repeat field and aerial photograph mapping and some detailed observations over a period of 20 years. These show major variations, some of which appear to be related to discharge fluctuations. The differing impact of the highest floods within the period of record is demonstrated. The state of the system, particularly the degree of meander development, is of crucial importance. A seasonal difference in the impact of high flow events is also detected. Relationships between morphological changes and discharge characteristics are complex and the effects of other factors, particularly growing season and ramped effects of growth of vegetation may be significant. Key words channel changes; channel migration; discharge fluctuations; erosion; floods; hydromorphology; river meanders; temperature changes INTRODUCTION Understanding of the response of river channels to climate and land-use changes has become even more important under scenarios of global warming and with new legislation, particularly the EU Water Framework Directive (WFD). Sediment movement takes place almost entirely in peak flow events but these may be of varying magnitude depending on the hydrological regime and the size range of sediment available in the system. It is primarily coarse sediment, from sand size upwards, that contributes to channel morphology. Meandering rivers are mainly developed in the flood plain zones of fluvial systems. Within these zones, the main source of coarse sediment to the channel system is erosion of the banks of the flood plains themselves, with occasional contributions from valley walls. Sediment supply from tributaries is localized in input. It is generally assumed that sediment has the potential to move down through the system. However, recent analyses have suggested that movement of the coarsest material may be much more localized in some systems and that there is not necessarily high connectivity between reaches, or that it may be operational only in very extreme events. (Fryirs & Brierley, 2001; Harvey, 2002; Hooke, 2003a). The incidence and impact of flood events in meandering rivers is examined here, through evidence of channel changes at a variety of spatial and temporal scales, to assess the conditions under which erosion and deposition take place and thus hydromorphological responses are brought about. The 128 analysis focuses on the Rivers Bollin a 1995a,b) with additional examples from and the River Wey in southeast England (H relationships are not simple feedback effects must be taken into account. HYDROLOGICAL VARATIONS The occurrence of peak flows has been analys common perception is that the climate change scenarios. The annual peak Janet Hooke are shown in Fig. 1 for four rivers from di Rivers Bollin and Dane in Cheshire, northwe in east Wales, and the River and that the complexity nd Dane in northwest England (Hooke 1987, differing patterns, one showing a slight dow the River Severn in Wales (Hooke a decrease and the others no tr of destabilization, there seem 1960s was even more extreme hydrologically ooke, 2002). The examples show that the rivers seem to show greater variation up unt reasons for this on each river. Inferences analyses based on 1970–2000, can be very differe incidence of flooding is in (a)a) Wey at Tilford in Surrey, southeast England. These do have of meander behaviour and 70 ed for a range of rivers in England. The 60 flows for a standardized period, 1960–1999 50 end. However, far from the most recent period being one 40 s to be some evidence to s fferent regions of England and Wales, the et al 30 st England, the River Severn at Abermule 20 ., 1994), Discharge-cumecs10 creasing and will do so under 0 nward trend in annual maximum flow, one (cc) 1960 ) 1963 700 1966 600 1969 il the earlyon some 1970s. rivers. Ther Three out of the four 500 1972 made on data that exclude this period, e.g. 400 300 1975 200 1978 uggest that the Discharge100 -cumecs 1981 nt from analyses that include it. 1984 (b) 0 b) 1987 1960 140 1990 120 1963 1993 100 1966 1996 80 e may be particularperiod of the 1969 1999 60 MEANDER CHANGES 1972 40 Discharge - cumecs 1975 20 Fig. 1 Evidence of the dynamics ofCheshire; some of(b) thesRiver,1978 Dane, Rudheath, Cheshi 0 1981 mainly on aerial photographic(d) data River and Wey, photogrammetric Tilford, Surrey. mapping. Additionally, the 1960 Variations1 9in84 annual peak discharge: (a) River Bollin,(d)d) Dunham Massey, 1963 1987 90 1966 1990 80 1969 1993 70 60 1972 1996 50 1975 1999 40 30 1978 Discharge20 - cumecs 10 1981 0 1984 1960 1987 1963 1990 1966 1993 1969 1996 1972 1999 re; (c) River Severn, Abermule, Wales; 1975 e streams is exemplif 1978 1981 1984 1987 1990 1993 1996 1999 ied in Fig. 2, based Hydromorphological adjustment in meandering river systems 129 (a) (b) (c) Fig. 2 Examples of channel changes: (a) River Dane, (b) River Severn, and (c) River Bollin. 130 Janet Hooke Bollin and Dane in northwest England have been mapped and monitored annually for the last 25 years. The Bollin and Dane and Caersws section of the River Severn are highly dynamic and respond readily to flood events, with active erosion and deposition usually occurring several times a year, as was shown earlier by intensive measure- ments of bank erosion on several dynamic meandering streams in southwest England (Hooke, 1979). Likewise, intensive measurements of movement of coarse material using tracing techniques on the River Yarty in Devon, southwest England, show that large cobbles are transported in events that occur on average several times a year (Barker, 2000). Thus the thresholds in relation to peak flow for coarse sediment transport are relatively low. Rates of erosion on the most active meander bends of these systems are of the order of 0.5–1.0 m year-1 and it is these bends which lead the hydromorphological response. Such systems are highly sensitive. RELATIONSHIPS OF CHANGE TO PEAK FLOWS The relationships between channel change or processes and the magnitude and frequency of peak flows have been analysed in various ways. Measurements of rates of erosion and of channel width derived from aerial photographs for available epochs are tabulated together with data on average mean annual flood and number of Peaks over Threshold (POT). Care must be taken in comparability of data but distinct differences in channel width are apparent for some epochs on some rivers (Table 1(a)). Data on areas or erosion and deposition derived from GIS analysis of the changes on the Dane and Severn show changes in the ratios of erosion and deposition (Table 1(b)). However, data on rates of movement of meanders of different type (Table 1(c)) show that this also has a profound influence on actual behaviour at any time, the different types of bend evolving and changing in different ways (Hooke, 1987). For the River Dane, annual observations have also been made of the occurrence of bank erosion, locations and amounts of deposition on bars, and calibre of sediment deposited on bars. These data, from a reach of ~10 km and comprising ~80 meander bends, have been amalgamated into indices of erosion and deposition. Sediment calibre is assessed by percentage of bars with dominantly cobble size material deposited. The annual data show considerable variability as would be expected and some general relationships but the correlation coefficients indicate that the occurrence of peak flows is not the complete explanation (Fig. 3). IMPACT OF LARGE FLOODS Much fluvial literature documents the impacts of extreme flood events and there is much discussion of the role of different magnitude–frequency events. The 2000–2001 peak flows in Britain were the highest on record for several decades on some rivers and were thought to have had a large impact. Comparison of both the magnitude– frequency of the event and the morphological impacts on the two neighbouring rivers, the Bollin and Dane, which are very similar in morphology and behaviour, is very instructive. On the River Dane, gauge records from 1949 show that the peak flow in November 2000 was the highest since records began. However, detailed ground field Hydromorphological adjustment in meandering river systems 131 Table 1(a) Changes in width of channels. River Dane 1984 1996 2001 Width (m) 18.6 15.7 17.2 Last peak Q (m3 s-1) 79.4 20 130.7 River Bollin 1970 1984 2001 Width (m) 10.3 8.0 9.85 Last peak Q (m3 s-1) 39.56 38.91 48.65 River Severn 1973 1984 1992 Width (m) 21.0 25.5 26.5 Last peak Q (m3 s-1) 131.18 207.11 230.97 Table 1(b) Areas of change in relation to discharge characteristics for period. River Dane 1984–1996 1996–2001 Erosion (m2) 43 122 31 735 Deposition (m2) 60 779 24 490 Erosion & Deposition (m2) 89 060 104 074 Ratio erosion /deposition 0.71 1.30 Mean annual max Q (m3 s-1) 77.4 88.9 Mean no. peaks 6.7 15.9 River Severn 1973–1984 1984–1992 Erosion (m2) 7785 15 089 Deposition (m2) 14 779 11 385 Erosion & deposition (m2) 1716 5821 Ratio erosion /deposition 0.53 1.33 Mean annual max Q (m3 s-1) 174.85 192.06 Mean no.